To change your style system options, go to http://www.insanejournal.com/customize/. Select either S1 or S2 from the drop-down box, and hit the "Change" button.
If you want to customize your journal using S2, you can do so at http://www.insanejournal.com/customize/ as well. Once you have selected a S2 layout to use, you will see the customization options offered by that layout.
You can switch your journal between S1 and S2, and each style system's settings will be saved. This means that you can choose the other style system, and if you decide that you don't like it, you can switch back and your previous customizations will be visible again.